1. Field of the Invention
The present invention relates to a PCM (Pulse Code Modulation) system, and more particularly to a correction for a signal lacking some data in the PCM transmission such as DPCM (differential PCM) in which a differential data is transmitted between samples transmitted. Here, the means transmission in a wide sense, including through a modulation/demodulation system and a recording/reproducing system etc., as well as through wireless or wire transmission lines. Accordingly, a "transmission path" means transmission/signalling lines, modulation/demodulation systems, and recording/reproducing systems, etc., including combinations thereof.
2. Prior Art
For example, in the DPCM transmission, a transmission error may be produced, which is incapable of being corrected by what is called error correction. In this instance, the signal in which an error signal is included is properly subjected to any correction for removing the error signal to obtain the original signal.
A sample case will be considered, in which, upon transmitting a sinusoidal signal as shown in FIG. 1, the signal is sampled for transmission at different points of A to D as in FIG. 1. Provided that the data is lost in part in the course of the transmission via a transmission system and is incapable of being retrieved at points B and C on the reception side, some correction therefore is needed. Assuming that a data is held just before it is partly lost (hereinafter referred to as "pre-hold") (when, in demodulation of DPCM, difference data are accumulated and integrated, partial lack of the difference data usually causes the "pre-hold"), this data provides points B' and C' on the reception side, as shown in FIG. 1. Then, provided that a proper difference data is yielded, it is added to the previous one, taking the point C' as its base. Accordingly, a point D of the original data provides a point D' as shown in FIG. 1 on the reception side, and allows the DC level of the signal to be sharply shifted. The amount of the shift of the DC level is equal to the accumulation of the lacked or deficient data values. In addition, with a dynamic range DR of a demodulation system, or a later stage regeneration system provided, as shown in FIG. 1 for example, the data at point D' is completely over the dynamic range DR, so that the demodulation/regeneration system is saturated, allowing a regenerated signal to be clipped.
To solve the problem described above, it is considered that an original sample data, not a difference data, is inserted for every plurality of prescribed samples, and thereby an (accumulated) integrated data on the reception stage is refreshed. It can be said in this instance that the inserted original sample data is an absolute data representative of an absolute value in comparison with the difference data corresponding to a relative value between respective samples. The absolute data of this type serves in the same way as transmission data employed in the ordinary PCM, i.e., linear PCM. An example of such a system will be described as follows with reference to FIG. 2. In this instance, for example, an 8-bit compressed difference data is obtained from a 14-bit sample data and is used for transmission. A 14-bit absolute data is added to the transmission data after every 16 samples of 8-bit difference data. The difference data are accumulated and integrated on the reception side to yield a regenerated signal, and an integrated difference data is refreshed with the absolute data each time the difference data is received.
With such an arrangement, even if an error is produced in the course of the transmission, the integrated value is corrected to a proper value each time the absolute data is supplied, and the error produced has no effect on the succeeding data. However, the original sample data, in this instance, a 14-bit data as described above, is compressed to an 8-bit difference data and transmitted, so that the above-described absolute data requires 14 bits, i.e., the bit number of the original sample data. Thus, the present system suffers from the problem that, although the original sample data is compressed to the 8-bit difference data to improve the transmission efficiency, this transmission efficiency is lowered by a fraction of 14 bits of the absolute data and a different kind of data having different bit numbers are composed, thereby causing a circuit for data separation on the reception side to be complicated. In addition, it is assumed in the signal waveform shown in FIG. 3 that an illustrated point F data is lost and an illustrated E point is pre-held. Thereupon, a regenerated signal for F.fwdarw.G.fwdarw.H changes to F'.fwdarw.G'.fwdarw.H' as shown in the FIG. 3. Further, the regenerated signal waveform, if refreshed at point H, changes to E.fwdarw.F'.fwdarw.G'.fwdarw.H'.fwdarw.H. Accordingly, the regenerated signal waveform is discontinuous at two portions between E.about.H' and between H.about.H'. Namely, the prior PCM transmission system suffers from the drawback that the regenerated signal waveform has many distortion components.